GCC 4.9.x build bootstrap comparison differences

[Matt Godbolt <matt at godbolt dot org>]

Hi all,

I've been building GCC from source for GCC Explorer[1]. To that end
I've modified a script I've used at work for many years to build GCC.
The result is a build script[2], a patch[3] (to get in-tree binutils
working on 4.x and 5.x), and a Docker container that allows me a
repeatable build of GCC.

The script uses an in-tree build of the latest binutils (2.27). It
correctly builds GCC 6.x and 5.x, but when configured to build 4.9.x I
get a failure comparing stages 2 and 3:

    Comparing stages 2 and 3
    warning: gcc/cc1plus-checksum.o differs
    warning: gcc/cc1-checksum.o differs
    Bootstrap comparison failure!
    gold/aarch64.o differs
    Makefile:28237: recipe for target 'compare' failed

I've tried with 4.9.0 through 4.9.4 but to no avail.

Can anyone offer any suggestions as to what might be going on? For
anyone interested in poking about in an instance and doesn't mind
using docker, one can get a shell into a build instance with `docker
run -ti mattgodbolt/gcc-builder bash` - the 'build.sh' in there
expects a GCC version number. Of course, one can also take a look at
the script source[2] which has all the config options I'm passing to
configure. Probably the interesting ones are LTO, linker build id and
gold (given the error message).

Thanks in advance for any help offered!

Regards, Matt

[1]: https://gcc.godbolt.org/
[2]: https://github.com/mattgodbolt/gcc-explorer-image/blob/master/gcc/build/build.sh
[3]: https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01233.html

-- 
Matt